Black holes once existed exclusively in the realm of theory, but astronomers have become increasingly adept at spotting the telltale signs of objects that are hard to spot due to the fact that they suck in any light that happens to cross an event horizon. Black holes have fallen into two separate categories: relatively small ones formed by the collapse of massive stars, and supermassive ones, which appear to lie at the hearts of most galaxies. When provided enough fuel in the form of interstellar gas, these black holes power quasars (also termed active galactic nuclei) but, deprived of input, they tend to sit quietly, much like the one at the center of the Milky Way.

The presence of black holes at the center of galaxies has been made a bit more intriguing by the fact that astronomers have reached the conclusion that many galaxies have been built by mergers of smaller ones. This would suggest that once the chaos of the initial collision settles down, the resulting galaxy should contain two black holes, with gravitational attraction potentially prompting a collision. Today's issue of Nature describes what appears to be the best evidence yet for a supermassive black hole binary, one in which the two objects appear on course for an inevitable collision.

Finding it wasn't exactly easy. The authors relied on data that had been gathered by the Sloan Digital Sky Survey, which currently contains over 900,000 galaxies. The authors selected a subset of the data that contains over 17,000 quasars, and sifted it using a principal components analysis that should identify outliers with unusual spectral properties. Out of that massive data set, a grand total of two objects were identified that appeared to emit light with more than one redshift.